Neurotrophin and neurotrophin receptor mRNA expression in developing inner ear

Differentiation of Spiral Ganglion Neurons from Human Dental Pulp Stem Cells: A Further Step towards Autologous Auditory Nerve Recovery

The degeneration of spiral ganglion neurons (SGNs), which convey auditory signals from hair cells to the brain, can be a primary cause of sensorineural hearing loss (SNHL) or can occur secondary to hair cell loss. Emerging therapies for SNHL include the replacement of damaged SGNs using stem cell-derived otic neuronal progenitors (ONPs). However, the availability of renewable, accessible, and patient-matched sources of human stem cells is a prerequisite for successful replacement of the auditory nerve. In this study, we derived ONP and SGN-like cells by a reliable and reproducible stepwise guidance differentiation procedure of self-renewing human dental pulp stem cells (hDPSCs). This in vitro differentiation protocol relies on the modulation of BMP and TGFβ pathways using a free-floating 3D neurosphere method, followed by differentiation on a Geltrex-coated surface using two culture paradigms to modulate the major factors and pathways involved in early otic neurogenesis. Gene and protein expression analyses revealed efficient induction of a comprehensive panel of known ONP and SGN-like cell markers during the time course of hDPSCs differentiation. Atomic force microscopy revealed that hDPSC-derived SGN-like cells exhibit similar nanomechanical properties as their in vivo SGN counterparts. Furthermore, spiral ganglion neurons from newborn rats come in close contact with hDPSC-derived ONPs 5 days after co-culturing. Our data demonstrate the capability of hDPSCs to generate SGN-like neurons with specific lineage marker expression, bipolar morphology, and the nanomechanical characteristics of SGNs, suggesting that the neurons could be used for next-generation cochlear implants and/or inner ear cell-based strategies for SNHL.

Relationship Between Serum Brain-Derived Neurotrophic Factor and Neurotrophin-3 Levels and Hearing Thresholds in Patients With Age-Related Hearing Loss

BACKGROUND

Age-related hearing loss (ARHL) is a general term used to describe the sensorineural type of hearing loss occurring in both ears in older adults. Neurotrophins are the most promising candidates for supporting the auditory nerve by increasing neuronal survival. This study aimed to help elucidate the pathophysiology of ARHL by determining whether any relationship exists between brain-derived neurotrophic factor (BDNF) and neurotrophin-3 (NT-3) levels in serum samples from patients diagnosed with ARHL.

MATERIALS AND METHOD

Seventy-seven individuals, a study group of 41 patients diagnosed with ARHL, and a control group of 36 participants without hearing loss were evaluated. Serum samples were collected and used to measure serum BDNF and NT-3 levels with the new Nepenthe enzyme-linked immunosorbent assay method.

RESULTS

Median pure-tone average results in the 2000, 4000, and 6000 Hz ranges were 52.5 (44.3-67.3) dB HL in the ARHL group and 13.5 (11.1-17.1) dB HL in the control group. The difference was statistically significant (p = .001). Although NT-3 and BDNF levels were both lower in ARHL patients than in participants without hearing loss, only the BDNF levels were significantly (p = .002) lower. Mean left and right ear word recognition scores were also lower in ARHL patients than in control groups. The ARHL group was further divided into two subgroups based on word recognition scores to evaluate significant differences in BDNF and NT-3 levels. No statistically significant difference was observed in BDNF and NT-3 levels between these subgroups. However, there was a significant difference in word recognition scores.

CONCLUSIONS

Low BDNF levels in the ARHL group suggest that BDNF may play a role in the pathogenesis of ARHL. Patients with low (ARHL1) and high (ARHL2) word recognition scores were compared for the first time in the literature in terms of BDNF and NT-3 levels. However, the results were not statistically significant. This article is a preliminary study and was written to provide guidance for our next comprehensive project.

The BDNF/TrkB Neurotrophin System in the Sensory Organs of Zebrafish

The brain-derived neurotrophic factor (BDNF) was discovered in the last century, and identified as a member of the neurotrophin family. BDNF shares approximately 50% of its amino acid with other neurotrophins such as NGF, NT-3 and NT-4/5, and its linear amino acid sequences in zebrafish (Danio rerio) and human are 91% identical. BDNF functions can be mediated by two categories of receptors: p75NTR and Trk. Intriguingly, BDNF receptors were highly conserved in the process of evolution, as were the other NTs’ receptors. In this review, we update current knowledge about the distribution and functions of the BDNF-TrkB system in the sensory organs of zebrafish. In fish, particularly in zebrafish, the distribution and functions of BDNF and TrkB in the brain have been widely studied. Both components of the system, associated or segregated, are also present outside the central nervous system, especially in sensory organs including the inner ear, lateral line system, retina, taste buds and olfactory epithelium.

Mechanism and Prevention of Spiral Ganglion Neuron Degeneration in the Cochlea

Sensorineural hearing loss (SNHL) is one of the most prevalent sensory deficits in humans, and approximately 360 million people worldwide are affected. The current treatment option for severe to profound hearing loss is cochlear implantation (CI), but its treatment efficacy is related to the survival of spiral ganglion neurons (SGNs). SGNs are the primary sensory neurons, transmitting complex acoustic information from hair cells to second-order sensory neurons in the cochlear nucleus. In mammals, SGNs have very limited regeneration ability, and SGN loss causes irreversible hearing loss. In most cases of SNHL, SGN damage is the dominant pathogenesis, and it could be caused by noise exposure, ototoxic drugs, hereditary defects, presbycusis, etc. Tremendous efforts have been made to identify novel treatments to prevent or reverse the damage to SGNs, including gene therapy and stem cell therapy. This review summarizes the major causes and the corresponding mechanisms of SGN loss and the current protection strategies, especially gene therapy and stem cell therapy, to promote the development of new therapeutic methods.

Neuronal development in the cochlea of a nonhuman primate model, the common marmoset

Precise cochlear neuronal development is vital to hearing ability. Understanding the developmental process of the spiral ganglion is useful for studying hearing loss aimed at aging or regenerative therapy. Although interspecies differences have been reported between rodents and humans, to date, most of our knowledge about the development of cochlear neuronal development has been obtained from rodent models because of the difficulty in using human fetal samples in this field. In this study, we investigated cochlear neuronal development in a small New World monkey species, the common marmoset (Callithrix jacchus). We examined more than 25 genes involved in the neuronal development of the cochlea and described the critical developmental steps of these neurons. We also revealed similarities and differences between previously reported rodent models and this primate animal model. Our results clarified that this animal model of cochlear neuronal development is more similar to humans than rodents and is suitable as an alternative for the analysis of human cochlear development. The time course established in this report will be a useful tool for studying primate‐specific neuronal biology of the inner ear, which could eventually lead to new treatment strategies for human hearing loss.

Neurotrophin gene therapy to promote survival of spiral ganglion neurons after deafness

Hearing impairment is a major health and economic concern worldwide. Currently, the cochlear implant (CI) is the standard of care for remediation of severe to profound hearing loss, and in general, contemporary CIs are highly successful. But there is great variability in outcomes among individuals, especially in children, with many CI users deriving much less or even marginal benefit. Much of this variability is related to differences in auditory nerve survival, and there has been substantial interest in recent years in exploring potential therapies to improve survival of the cochlear spiral ganglion neurons (SGN) after deafness. Preclinical studies using osmotic pumps and other approaches in deafened animal models to deliver neurotrophic factors (NTs) directly to the cochlea have shown promising results, especially with Brain-Derived Neurotrophic Factor (BDNF). More recent studies have focused on the use of NT gene therapy to force expression of NTs by target cells within the cochlea. This could provide the means for a one-time treatment to promote long-term NT expression and improve neural survival after deafness. This review summarizes the evidence for the efficacy of exogenous NTs in preventing SGN degeneration after hearing loss and reviews the animal research to date suggesting that NT gene therapy can elicit long-term NT expression in the cochlea, resulting in significantly improved SGN and radial nerve fiber survival after deafness. In addition, we discuss NT gene therapy in other non-auditory applications and consider some of the remaining issues with regard to selecting optimal vectors, timing of treatment, and place/method of delivery, etc. that must be resolved prior to considering clinical application.

New molecular therapies for the treatment of hearing loss

An estimated 466 million people suffer from hearing loss worldwide. Sensorineural hearing loss is characterized by degeneration of key structures of the sensory pathway in the cochlea such as the sensory hair cells, the primary auditory neurons and their synaptic connection to the hair cells - the ribbon synapse. Various strategies to protect or regenerate these sensory cells and structures are the subject of intensive research. Yet despite recent advances in our understandings of the capacity of the cochlea for repair and regeneration there are currently no pharmacological or biological interventions for hearing loss. Current research focusses on localized cochlear drug, gene and cell-based therapies. One of the more promising drug-based therapies is based on neurotrophic factors for the repair of the ribbon synapse after noise exposure, as well as preventing loss of primary auditory neurons and regrowth of the auditory neuron fibers after severe hearing loss. Drug therapy delivery technologies are being employed to address the specific needs of neurotrophin and other therapies for hearing loss that include the need for high doses, long-term delivery, localised or cell-specific targeting and techniques for their safe and efficacious delivery to the cochlea. Novel biomaterials are enabling high payloads of drugs to be administered to the cochlea with subsequent slow-release properties that are proving to be beneficial for treating hearing loss. In parallel, new gene therapy technologies are addressing the need for cell specificity and high efficacy for the treatment of both genetic and acquired hearing loss with promising reports of hearing recovery. Some biomaterials and cell therapies are being used in conjunction with the cochlear implant ensuring therapeutic benefit to the primary neurons during electrical stimulation. This review will introduce the auditory system, hearing loss and the potential for repair and regeneration in the cochlea. Drug delivery to the cochlea will then be reviewed, with a focus on new biomaterials, gene therapy technologies, cell therapy and the use of the cochlear implant as a vehicle for drug delivery. With the current pre-clinical research effort into therapies for hearing loss, including clinical trials for gene therapy, the future for the treatment for hearing loss is looking bright.

The feasibility of an encapsulated cell approach in an animal deafness model

For patients with profound hearing loss a cochlear implant (CI) is the only treatment today. The function of a CI depends in part of the function and survival of the remaining spiral ganglion neurons (SGN). It is well known from animal models that inner ear infusion of neurotrophic factors prevents SGN degeneration and maintains electrical responsiveness in deafened animals. The purpose with this study was to investigate the effects of a novel encapsulated cell (EC) device releasing neurotrophic factors in the deafened guinea pig.

The results showed that an EC device releasing glial cell line-derived neurotrophic factor (GDNF) or brain-derived neurotrophic factor (BDNF) implanted for four weeks in deafened guinea pigs significantly preserved the SGNs and maintained their electrical responsiveness. There was a significant difference between BDNF and GDNF in favour of GDNF. This study, demonstrating positive structural and functional effects in the deafened inner ear, suggests that an implanted EC device releasing biologically protective substances offers a feasible approach for treating progressive hearing impairment.

Structural and Ultrastructural Changes to Type I Spiral Ganglion Neurons and Schwann Cells in the Deafened Guinea Pig Cochlea

Sensorineural hearing loss is commonly caused by damage to cochlear sensory hair cells. Coinciding with hair cell degeneration, the peripheral fibres of type I spiral ganglion neurons (SGNs) that normally form synaptic connections with the inner hair cell gradually degenerate. We examined the time course of these degenerative changes in type I SGNs and their satellite Schwann cells at the ultrastructural level in guinea pigs at 2, 6, and 12 weeks following aminoglycoside-induced hearing loss. Degeneration of the peripheral fibres occurred prior to the degeneration of the type I SGN soma and was characterised by shrinkage of the fibre followed by retraction of the axoplasm, often leaving a normal myelin lumen devoid of axoplasmic content. A statistically significant reduction in the cross-sectional area of peripheral fibres was evident as early as 2 weeks following deafening (p < 0.001, ANOVA). This was followed by a decrease in type I SGN density within Rosenthal's canal that was statistically significant 6 weeks following deafening (p < 0.001, ANOVA). At any time point examined, few type I SGN soma were observed undergoing degeneration, implying that once initiated, soma degeneration was rapid. While there was a significant reduction in soma area as well as changes to the morphology of the soma, the ultrastructure of surviving type I SGN soma appeared relatively normal over the 12-week period following deafening. Satellite Schwann cells exhibited greater survival traits than their type I SGN; however, on loss of neural contact, they reverted to a non-myelinating phenotype, exhibiting an astrocyte-like morphology with the formation of processes that appeared to be searching for new neural targets. In 6- and 12-week deafened cochlea, we observed cellular interaction between Schwann cell processes and residual SGNs that distorted the morphology of the SGN soma. Understanding the response of SGNs, Schwann cells, and the complex relationship between them following aminoglycoside deafening is important if we are to develop effective therapeutic techniques designed to rescue SGNs.

Exposure to diphtheria toxin during the juvenile period impairs both inner and outer hair cells in C57BL/6 mice

Diphtheria toxin (DT) administration into transgenic mice that express the DT receptor (DTR) under control of specific promoters is often used for cell ablation studies in vivo. Because DTR is not expressed in mice, DT injection has been assumed to be nontoxic to cells in vivo. In this study, we demonstrated that DT application during the juvenile stage leads to hearing loss in wild-type mice. Auditory brainstem response measurement showed severe hearing loss in C57BL/6 mice administered DT during the juvenile period, and the hearing loss persisted into adulthood. However, ototoxicity did not occur when DT was applied on postnatal day 28 or later. Histological studies demonstrated that hearing loss was accompanied by significant degeneration of inner and outer hair cells (HCs), as well as spiral ganglion neurons. Scanning electron microscopy showed quick degeneration of inner HCs within 3days and gradual degeneration of outer HCs within 1week. These results demonstrated that DT has ototoxic action on C57BL/6 mice during the juvenile period, but not thereafter, and the hearing loss was due to degeneration of inner and outer HCs by unknown DT-related mechanisms.

Effects of brain-derived neurotrophic factor (BDNF) on the cochlear nucleus in cats deafened as neonates

Many previous studies have shown significant neurotrophic effects of intracochlear delivery of BDNF in preventing degeneration of cochlear spiral ganglion (SG) neurons after deafness in rodents and our laboratory has shown similar results in developing cats deafened prior to hearing onset. This study examined the morphology of the cochlear nucleus (CN) in a group of neonatally deafened cats from a previous study in which infusion of BDNF elicited a significant improvement in survival of the SG neurons. Five cats were deafened by systemic injections of neomycin sulfate (60 mg/kg, SQ, SID) starting one day after birth, and continuing for 16-18 days until auditory brainstem response (ABR) testing demonstrated profound bilateral hearing loss. The animals were implanted unilaterally at about 1 month of age using custom-designed electrodes with a drug-delivery cannula connected to an osmotic pump. BDNF (94 μg/ml; 0.25 μl/hr) was delivered for 10 weeks. The animals were euthanized and studied at 14-23 weeks of age. Consistent with the neurotrophic effects of BDNF on SG survival, the total CN volume in these animals was significantly larger on the BDNF-treated side than on the contralateral side. However, total CN volume, both ipsi- and contralateral to the implants in these deafened juvenile animals, was markedly smaller than the CN in normal adult animals, reflecting the severe effects of deafness on the central auditory system during development. Data from the individual major CN subdivisions (DCN, Dorsal Cochlear Nucleus; PVCN, Posteroventral Cochlear Nucleus; AVCN, Anteroventral Cochlear Nucleus) also were analyzed. A significant difference was observed between the BDNF-treated and control sides only in the AVCN. Measurements of the cross-sectional areas of spherical cells showed that cells were significantly larger in the AVCN ipsilateral to the implant than on the contralateral side. Further, the numerical density of spherical cells was significantly lower in the AVCN ipsilateral to the implant than on the contralateral side, consistent with the larger AVCN volume observed with BDNF treatment. Together, findings indicate significant neurotrophic effects of intracochlear BDNF infusion on the developing CN.

Firing frequency and entrainment maintained in primary auditory neurons in the presence of combined BDNF and NT3

Primary auditory neurons rely on neurotrophic factors for development and survival. We previously determined that exposure to brain-derived neurotrophic factor (BDNF) and neurotrophin-3 (NT3) alters the activity of hyperpolarization-activated currents (Ih) in this neuronal population. Since potassium channels are sensitive to neurotrophins, and changes in Ih are often accompanied by a shift in voltage-gated potassium currents (IK), this study examined IK with exposure to both BDNF and NT3 and the impact on firing entrainment during high frequency pulse trains. Whole-cell patch-clamp recordings revealed significant changes in action potential latency and duration, but no change in firing adaptation or total outward IK. Dendrotoxin-I (DTX-I), targeting voltage-gated potassium channel subunits KV1.1 and KV1.2, uncovered an increase in the contribution of DTX-I sensitive currents with exposure to neurotrophins. No difference in Phrixotoxin-1 (PaTX-1) sensitive currents, mediated by KV4.2 and KV4.3 subunits, was observed. Further, no difference was seen in firing entrainment. These results show that combined BDNF and NT3 exposure influences the contribution of KV1.1 and KV1.2 to the low voltage-activated potassium current (IKL). Whilst this is accompanied by a shift in spike latency and duration, both firing frequency and entrainment to high frequency pulse trains are preserved.

Postnatal expression of neurotrophic factors accessible to spiral ganglion neurons in the auditory system of adult hearing and deafened rats

Spiral ganglion neurons (SGNs) receive input from cochlear hair cells and project from the cochlea to the cochlear nucleus. After destruction of hair cells with aminoglycoside antibiotics or noise, SGNs gradually die. It has been assumed that SGN death is attributable to loss of neurotrophic factors (NTFs) derived from hair cells or supporting cells in the organ of Corti (OC). We used quantitative PCR (qPCR) to assay NTF expression-neurotrophin-3 (NT-3), BDNF, GDNF, neurturin, artemin, and CNTF-in the OC and cochlear nucleus at various ages from postnatal day 0 (P0) to P90 in control hearing and neonatally deafened rats. NT-3, neurturin, and CNTF were most abundant in the postnatal hearing OC; CNTF and neurturin most abundant in the cochlear nucleus. In the OC, NT-3 and CNTF showed a postnatal increase in expression approximately concomitant with hearing onset. In rats deafened by daily kanamycin injections (from P8 to P16), surviving inner hair cells were evident at P16 but absent by P19, with most postsynaptic boutons lost before P16. NT-3 and CNTF, which normally increase postnatally, had significantly reduced expression in the OC of deafened rats, although CNTF was expressed throughout the time that SGNs were dying. In contrast, neurturin expression was constant, unaffected by deafening or by age. CNTF and neurturin expression in the cochlear nucleus was unaffected by deafening or age. Thus, NTFs other than NT-3 are available to SGNs even as they are dying after deafening, apparently conflicting with the hypothesis that SGN death is attributable to lack of NTFs.

Hemolysis in neonatal rats results in auditory impairment

CONCLUSION

This study suggests that hyperbilirubinemia in the neonatal rat can impair auditory function and induce peripheral nerve pathology by reducing neurofilament-positive cells in spiral ganglion neurons (SGNs). This finding indicates a potential connection between hyperbilirubinemia and auditory impairment.

OBJECTIVE

To establish a neonatal rat hyperbilirubinemia induced by hemolysis and assess the possible link between hyperbilirubinemia and auditory impairment.

METHODS

Wistar rats were divided into two groups - a bilirubin exposure group injected with phenylhydrazine hydrochloride at 7 and 28 days of age to induce hyperbilirubinemia, and a control group given saline. Auditory brainstem response (ABR) and distortion product otoacoustic emissions (DPOAEs) were determined to assess auditory function. Cochlea basilar membrane stretch preparations and cochlear frozen sections were examined for morphological changes in hair cells and SGNs.

RESULTS

At day 7, ABR wave I, III, and V latencies, and I-III, I-V interwave intervals (IWIs) in the experimental group were significantly prolonged compared with those in the control group. ABR thresholds were also elevated in the experimental group. We found no significant difference in DPOAEs in the bilirubin exposure group compared to the control group. The ABRs and DPOAEs in the experimental group were restored at age 28 days. Cochlear hair cells showed no signs of loss in either group; however, the total number of neurofilament-positive cells in SGNs was significantly reduced in the phenylhydrazine-treated animals.

Neurotrophin-3 regulates ribbon synapse density in the cochlea and induces synapse regeneration after acoustic trauma

Neurotrophin-3 (Ntf3) and brain derived neurotrophic factor (Bdnf) are critical for sensory neuron survival and establishment of neuronal projections to sensory epithelia in the embryonic inner ear, but their postnatal functions remain poorly understood. Using cell-specific inducible gene recombination in mice we found that, in the postnatal inner ear, Bbnf and Ntf3 are required for the formation and maintenance of hair cell ribbon synapses in the vestibular and cochlear epithelia, respectively. We also show that supporting cells in these epithelia are the key endogenous source of the neurotrophins. Using a new hair cell CreER(T) line with mosaic expression, we also found that Ntf3's effect on cochlear synaptogenesis is highly localized. Moreover, supporting cell-derived Ntf3, but not Bbnf, promoted recovery of cochlear function and ribbon synapse regeneration after acoustic trauma. These results indicate that glial-derived neurotrophins play critical roles in inner ear synapse density and synaptic regeneration after injury.

Electroacoustic stimulation: now and into the future

Cochlear implants have provided hearing to hundreds of thousands of profoundly deaf people around the world. Recently, the eligibility criteria for cochlear implantation have been relaxed to include individuals who have some useful residual hearing. These recipients receive inputs from both electric and acoustic stimulation (EAS). Implant recipients who can combine these hearing modalities demonstrate pronounced benefit in speech perception, listening in background noise, and music appreciation over implant recipients that rely on electrical stimulation alone. The mechanisms bestowing this benefit are unknown, but it is likely that interaction of the electric and acoustic signals in the auditory pathway plays a role. Protection of residual hearing both during and following cochlear implantation is critical for EAS. A number of surgical refinements have been implemented to protect residual hearing, and the development of hearing-protective drug and gene therapies is promising for EAS recipients. This review outlines the current field of EAS, with a focus on interactions that are observed between these modalities in animal models. It also outlines current trends in EAS surgery and gives an overview of the drug and gene therapies that are clinically translatable and may one day provide protection of residual hearing for cochlear implant recipients.

Protection of spiral ganglion neurons from degeneration using small-molecule TrkB receptor agonists

Neurotrophins (NTs) play essential roles in the development and survival of neurons in PNS and CNS. In the cochlea, NTs [e.g., NT-3, brain-derived neurotrophic factor (BDNF)] are required for the survival of spiral ganglion neurons (SGNs). Preservation of SGNs in the cochlea of patients suffering sensorineural deafness caused by loss of hair cells is needed for the optimal performance of the cochlear implant. Directly applying exogenous BDNF into the cochlea prevents secondary degeneration of SGNs when hair cells are lost. However, a common translational barrier for in vivo applications of BDNF is the poor pharmacokinetics, which severely limits the efficacy. Here we report that 7,8-dihydroxyflavone and 7,8,3'-trihydroxyflavone, both small-molecule agonists of tyrosine receptor kinase B (TrkB), promoted SGN survival with high potency both in vitro and in vivo. These compounds increased the phosphorylated TrkB and downstream MAPK and protected the SGNs in a TrkB-dependent manner. Their applications in the bulla of conditional connexin26 null mice offered significant protection for SGN survival. The function of survived SGNs was assessed by measuring evoked action potentials (APs) in vitro and electrically evoked auditory brainstem response (eABR) thresholds in vivo. APs were reliably evoked in cultured single SGNs treated with the compounds. In addition, eABR thresholds measured from the treated cochleae were significantly lower than untreated controls. Our findings suggest that these novel small-molecule TrkB agonists are promising in vivo therapeutic agents for preventing degeneration of SGNs.

Changes in responsiveness of rat spiral ganglion neurons to neurotrophins across age: differential regulation of survival and neuritogenesis

Developmental changes in responsiveness of rat spiral ganglion neurons (SGNs) to neurotrophin-3 (NT-3) and brain-derived neurotrophic factor (BDNF) were examined using an explant culture system. Spiral ganglion (SG) explants at embryonic Day 18 (E18), postnatal Day 0 (P0), P5, P10 and P20 were cultured with the addition of either NT-3 or BDNF at various concentrations (0.1-100 ng/ml) and analyzed the dose-response characteristics of three parameters: SGN survival, the number of neurites emanating from the explants and the length of neurite extension. In E18 cultures, SGN survival and neurite number were enhanced more strongly by NT-3 than by the BDNF. As the explants became more mature, the effects of NT-3 decreased, whereas those of BDNF increased, peaking at P0. Although the intrinsic capacity of SGNs to produce and extend neurites declined considerably by P20, they still retained the capacity to respond to both NT-3 and BDNF. These temporal patterns in responsiveness of SGNs to neurotrophins correspond well to the expression pattern of the two neurotrophins in cochlear sensory epithelium in vivo and also correlate with the time course of developmental events in SGNs such as cell death and the establishment of mature hair cell innervation patterns.

Thyroid hormone increases fibroblast growth factor receptor expression and disrupts cell mechanics in the developing organ of corti

Background

Thyroid hormones regulate growth and development. However, the molecular mechanisms by which thyroid hormone regulates cell structural development are not fully understood. The mammalian cochlea is an intriguing system to examine these mechanisms, as cellular structure plays a key role in tissue development, and thyroid hormone is required for the maturation of the cochlea in the first postnatal week.

Results

In hypothyroid conditions, we found disruptions in sensory outer hair cell morphology and fewer microtubules in non-sensory supporting pillar cells. To test the functional consequences of these cytoskeletal defects on cell mechanics, we combined atomic force microscopy with live cell imaging. Hypothyroidism stiffened outer hair cells and supporting pillar cells, but pillar cells ultimately showed reduced cell stiffness, in part from a lack of microtubules. Analyses of changes in transcription and protein phosphorylation suggest that hypothyroidism prolonged expression of fibroblast growth factor receptors, and decreased phosphorylated Cofilin.

Conclusions

These findings demonstrate that thyroid hormones may be involved in coordinating the processes that regulate cytoskeletal dynamics and suggest that manipulating thyroid hormone sensitivity might provide insight into the relationship between cytoskeletal formation and developing cell mechanical properties.

The convergence of cochlear implantation with induced pluripotent stem cell therapy

According to 2010 estimates from The National Institute on Deafness and other Communication Disorders, approximately 17% (36 million) American adults have reported some degree of hearing loss. Currently, the only clinical treatment available for those with severe-to-profound hearing loss is a cochlear implant, which is designed to electrically stimulate the auditory nerve in the absence of hair cells. Whilst the cochlear implant has been revolutionary in terms of providing hearing to the severe-to-profoundly deaf, there are variations in cochlear implant performance which may be related to the degree of degeneration of auditory neurons following hearing loss. Hence, numerous experimental studies have focused on enhancing the efficacy of cochlear implants by using neurotrophins to preserve the auditory neurons, and more recently, attempting to replace these dying cells with new neurons derived from stem cells. As a result, several groups are now investigating the potential for both embryonic and adult stem cells to replace the degenerating sensory elements in the deaf cochlea. Recent advances in our knowledge of stem cells and the development of induced pluripotency by Takahashi and Yamanaka in 2006, have opened a new realm of science focused on the use of induced pluripotent stem (iPS) cells for therapeutic purposes. This review will provide a broad overview of the potential benefits and challenges of using iPS cells in combination with a cochlear implant for the treatment of hearing loss, including differentiation of iPS cells into an auditory neural lineage and clinically relevant transplantation approaches.

Drug delivery to the inner ear

Bionic devices electrically activate neural populations to partially restore lost function. Of fundamental importance is the functional integrity of the targeted neurons. However, in many conditions the ongoing pathology can lead to continued neural degeneration and death that may compromise the effectiveness of the device and limit future strategies to improve performance. The use of drugs that can prevent nerve cell degeneration and promote their regeneration may improve clinical outcomes. In this paper we focus on strategies of delivering neuroprotective drugs to the auditory system in a way that is safe and clinically relevant for use in combination with a cochlear implant. The aim of this approach is to prevent neural degeneration and promote nerve regrowth in order to improve outcomes for cochlear implant recipients using techniques that can be translated to the clinic.

Combined application of brain-derived neurotrophic factor and neurotrophin-3 and its impact on spiral ganglion neuron firing properties and hyperpolarization-activated currents

Neurotrophins provide an effective tool for the rescue and regeneration of spiral ganglion neurons (SGNs) following sensorineural hearing loss. However, these nerve growth factors are also potent modulators of ion channel activity and expression, and in the peripheral auditory system brain-derived neurotrophic factor (BDNF) and neurotrophin-3 (NT3) have previously been shown to alter the firing properties of auditory neurons and differentially regulate the expression of some potassium channels in vitro. In this study we examined the activity of the hyperpolarization-mediated mixed-cation current (I(h)) in early post-natal cultured rat SGNs following exposure to combined BDNF and NT3. Whole-cell patch-clamp recordings made after 1 or 2 days in vitro revealed no change in the firing adaptation of neurons in the presence of BDNF and NT3. Resting membrane potentials were also maintained, but spike latency and firing threshold was subject to regulation by both neurotrophins and time in vitro. Current clamp recordings revealed an activity profile consistent with activation of the hyperpolarization-activated current. Rapid membrane hyperpolarization was followed by a voltage- and time-dependent depolarizing voltage sag. In voltage clamp, membrane hyperpolarization evoked a slowly-activating inward current that was reversibly blocked with cesium and inhibited by ZD7288. The amplitude and current density of I(h) was significantly larger in BDNF and NT3 supplemented cultures, but this did not translate to a significant alteration in voltage sag magnitude. Neurotrophins provided at 50 ng/ml produced a hyperpolarizing shift in the voltage-dependence and slower time course of I(h) activation compared to SGNs in control groups or cultured with 10 ng/ml BDNF and NT3. Our results indicate that combined BDNF and NT3 increase the activity of hyperpolarization-activated currents and that the voltage-dependence and activation kinetics of I(h) in SGNs are sensitive to changes in neurotrophin concentration. In addition, BDNF and NT3 applied together induce a decrease in firing threshold, but does not generate a shift in firing adaptation.

Gene therapy for cisplatin-induced ototoxicity: a systematic review of in vitro and experimental animal studies

OBJECTIVE

Ototoxicity is a frequent adverse event of cisplatin treatment. No therapy is currently available for cisplatin-induced ototoxicity. A systematic review of experimental animal studies and in vitro experiments was conducted to evaluate gene therapy as a potential future therapeutic option.

DATA SOURCES

Eligible studies were identified through searches of electronic databases Ovid MEDLINE, Ovid MEDLINE In-Process, Embase, PubMed, Biosis Previews, Scopus, ISI Web of Science, and The Cochrane Library.

STUDY SELECTION

Articles obtained from the search were independently reviewed by 2 authors using specific criteria to identify experimental animal studies and in vitro experiments conducted to evaluate gene therapy for cisplatin-induced ototoxicity. No restriction was applied to publication dates or languages.

DATA EXTRACTION

Data extracted included experiment type, cell type, species, targeted gene, gene expression, method, administration, inner ear site evaluated, outcome measures for cytotoxicity, and significant results.

RESULTS

Fourteen articles were included in this review. In vitro and in vivo experiments have been performed to evaluate the potential of gene expression manipulation for cisplatin-induced ototoxicity. Twelve different genes were targeted including NTF3, GDNF, HO-1, XIAP, Trpv1, BCL2, Otos, Nfe2l2, Nox1, Nox3, Nox4, and Ctr1. All of the included articles demonstrated a benefit of gene therapy on cytotoxicity caused by cisplatin.

CONCLUSION

Experimental animal studies and in vitro experiments have demonstrated the efficacy of gene therapy for cisplatin-induced ototoxicity. However, further investigation regarding safety, immunogenicity, and consequences of genetic manipulation in the inner ear tissues must be completed to develop future therapeutic options.

Ras/p38 and PI3K/Akt but not Mek/Erk signaling mediate BDNF-induced neurite formation on neonatal cochlear spiral ganglion explants

Neurotrophins participate in regulating the survival, differentiation, and target innervation of many neurons, mediated by high-affinity Trk and low-affinity p75 receptors. In the cochlea, spiral ganglion (SG) neuron survival is strongly dependent upon neurotrophic input, including brain-derived neurotrophic factor (BDNF), which increases the number of neurite outgrowth in neonatal rat SG in vitro. Less is known about signal transduction pathways linking the activation of neurotrophin receptors to SG neuron nuclei. In particular, the p38 and cJUN Kinase (JNK), mitogen-activated protein kinase (MAPK) pathways, which participate in JNK signaling in other neurons, have not been studied. We found that inhibition of Ras, p38, phosphatidyl inositol 3 kinase (PI3K) or Akt signaling reduced or eliminated BDNF mediated increase in number of neurite outgrowth, while inhibition of Mek/Erk had no influence. Inhibition of Rac/cdc42, which lies upstream of JNK, modestly enhanced BDNF induced formation of neurites. Western blotting implicated p38 and Akt signaling, but not Mek/Erk. The results suggest that the Ras/p38 and PI3K/Akt are the primary pathways by which BDNF promotes its effects. Activation of Rac/cdc42/JNK signaling by BDNF may reduce the formation of neurites. This is in contrast to our previous results on NT-3, in which Mek/Erk signaling was the primary mediator of SG neurite outgrowth in vitro. Our data on BDNF agree with prior results from others that have implicated PI3K/Akt involvement in mediating the effects of BDNF on SG neurons in vitro, including neuronal survival and neurite extension. However, the identification of p38 and JNK involvement is entirely novel. The results suggest that neurotrophins can exert opposing effects on SG neurons, the balance of competing signals influencing the generation of neurites. This competition could provide a potential mechanism for the control of neurite number during development.

Structure and development of cochlear afferent innervation in mammals

In mammals, sensorineural deafness results from damage to the auditory receptors of the inner ear, the nerve pathways to the brain or the cortical area that receives sound information. In this review, we first focused on the cellular and molecular events taking part to spiral ganglion axon growth, extension to the organ of Corti, and refinement. In the second half, we considered the functional maturation of synaptic contacts between sensory hair cells and their afferent projections. A better understanding of all these processes could open insights into novel therapeutic strategies aimed to re-establish primary connections from sound transducers to the ascending auditory nerve pathways.

Enhanced auditory neuron survival following cell-based BDNF treatment in the deaf guinea pig

Exogenous neurotrophin delivery to the deaf cochlea can prevent deafness-induced auditory neuron degeneration, however, we have previously reported that these survival effects are rapidly lost if the treatment stops. In addition, there are concerns that current experimental techniques are not safe enough to be used clinically. Therefore, for such treatments to be clinically transferable, methods of neurotrophin treatment that are safe, biocompatible and can support long-term auditory neuron survival are necessary. Cell transplantation and gene transfer, combined with encapsulation technologies, have the potential to address these issues. This study investigated the survival-promoting effects of encapsulated BDNF over-expressing Schwann cells on auditory neurons in the deaf guinea pig. In comparison to control (empty) capsules, there was significantly greater auditory neuron survival following the cell-based BDNF treatment. Concurrent use of a cochlear implant is expected to result in even greater auditory neuron survival, and provide a clinically relevant method to support auditory neuron survival that may lead to improved speech perception and language outcomes for cochlear implant patients.

Complex primary afferents: What the distribution of electrophysiologically-relevant phenotypes within the spiral ganglion tells us about peripheral neural coding

Spiral ganglion neurons are the first neural element of the auditory system. They receive precise synaptic signals which represent features of sound stimuli encoded by hair cell receptors and they deliver a digital representation of this information to the central nervous system. It is well known that spiral ganglion neurons are selectively responsive to specific sound frequencies, and that numerous structural and physiological specializations in the inner ear increase the quality of this tuning, beyond what could be accomplished by the passive properties of the basilar membrane. Further, consistent with what we know about other sensory systems, it is becoming clear that the parallel divergent innervation pattern of type I spiral ganglion neurons has the potential to encode additional features of sound stimuli. To date, we understand the most about the sub-modalities of frequency and intensity coding in the peripheral auditory system. Work reviewed herein will address the issue of how intrinsic electrophysiological features of the neurons themselves have the potential to contribute to the precision of coding and transmitting information about these two parameters to higher auditory centers for further processing.

Interaction of neurotrophin signaling with Bcl-2 localized to the mitochondria and endoplasmic reticulum on spiral ganglion neuron survival and neurite growth

Enhanced spiral ganglion neuron (SGN) survival and regeneration of peripheral axons following deafness will likely enhance the efficacy of cochlear implants. Overexpression of Bcl-2 prevents SGN death but inhibits neurite growth. Here we assessed the consequences of Bcl-2 targeted to either the mitochondria (GFP-Bcl-2-Maob) or the endoplasmic reticulum (ER, GFP-Bcl-2-Cb5) on cultured SGN survival and neurite growth. Transfection of wild-type GFP-Bcl-2, GFP-Bcl-2-Cb5, or GFP-Bcl-2-Maob increased SGN survival, with GFP-Bcl-2-Cb5 providing the most robust response. Paradoxically, expression of GFP-Bcl-2-Maob results in SGN death in the presence of neurotrophin-3 (NT-3) and brain-derived neurotrophic factor (BDNF), neurotrophins that independently promote SGN survival via Trk receptors. This loss of SGNs is associated with cleavage of caspase 3 and appears to be specific for neurotrophin signaling, insofar as coexpression of constitutively active mitogen-activated kinase kinase (MEKDeltaEE) or phosphatidyl inositol-3 kinase (P110), but not other prosurvival stimuli (e.g., membrane depolarization), also results in the loss of SGNs expressing GFP-Bcl-2-Maob. MEKDeltaEE and P110 promote SGN survival, whereas P110 promotes neurite growth to a greater extent than NT-3 or MEKDeltaEE. However, wild-type GFP-Bcl-2, GFP-Bcl-2-Cb5, and GFP-Bcl-2-Maob inhibit neurite growth even in the presence of neurotrophins, MEKDeltaEE, or P110. Historically, Bcl-2 has been thought to act primarily at the mitochondria to prevent neuronal apoptosis. Nevertheless, our data show that Bcl-2 targeted to the ER is more effective at rescuing SGNs in the absence of trophic factors. Additionally, Bcl-2 targeted to the mitochondria results in SGN death in the presence of neurotrophins. (c) 2010 Wiley-Liss, Inc.

Postnatal development of NT3 and TrkC in mouse ventral cochlear nucleus

In the developing nervous system, neurotrophin 3 (NT3) and brain-derived neurotrophic factor (BDNF) have been shown to interact with each other and with different parts of a neuron or glia and over considerable distances in time and space. The auditory system provides a useful model for analyzing these events, insofar as it is subdivided into well-defined groups of specific neuronal types that are readily related to each other at each stage of development. Previous work in our laboratory suggested that NT3 and its receptor TrkC in the mouse cochlear nucleus (CN) may be involved in directing neuronal migration and initial targeting of inputs from cochlear nerve axons in the embryo. NT3 is hard to detect soon after birth, but TrkC lingers longer. Here we found NT3 and TrkC around P8 and the peak around P30. Prominent in ventral CN, associated with globular bushy cells and stellate cells, they were localized to different subcellular sites. The TrkC immunostain was cytoplasmic, and that of NT3 was axonal and perisomatic. TrkC may be made by CN neurons, whereas NT3 has a cochlear origin. The temporal pattern of their development and the likelihood of activity-dependent release of NT3 from cochlear axons suggest that it may not be critical in early synaptogenesis; it may provide long-term trophic effects, including stabilization of synapses once established. Activity-related regulation could coordinate the supply of NT3 with inner ear activity. This may require interaction with other neurotrophins, such as BDNF.

Disorganized innervation and neuronal loss in the inner ear of Slitrk6-deficient mice

Slitrks are type I transmembrane proteins that share conserved leucine-rich repeat domains similar to those in the secreted axonal guidance molecule Slit. They also show similarities to Ntrk neurotrophin receptors in their carboxy-termini, sharing a conserved tyrosine residue. Among 6 Slitrk family genes in mammals, Slitrk6 has a unique expression pattern, with strong expression in the sensory epithelia of the inner ear. We generated Slitrk6-knockout mice and investigated the development of their auditory and vestibular sensory organs. Slitrk6-deficient mice showed pronounced reduction in the cochlear innervation. In the vestibule, the innervation to the posterior crista was often lost, reduced, or sometimes misguided. These defects were accompanied by the loss of neurons in the spiral and vestibular ganglia. Cochlear sensory epithelia from Slitrk6-knockout mice have reduced ability in promoting neurite outgrowth of spiral ganglion neurons. Indeed the Slitrk6-deficient inner ear showed a mild but significant decrease in the expression of Bdnf and Ntf3, both of which are essential for the innervation and survival of sensory neurons. In addition, the expression of Ntrk receptors, including their phosphorylated forms was decreased in Slitrk6-knockout cochlea. These results suggest that Slitrk6 promotes innervation and survival of inner ear sensory neurons by regulating the expression of trophic and/or tropic factors including neurotrophins from sensory epithelia.

Connexin30 null and conditional connexin26 null mice display distinct pattern and time course of cellular degeneration in the cochlea

Mutations in connexin26 (Cx26) and Cx30 are the most common cause of nonsyndromic inherited deafness in humans. To understand the underlying molecular mechanisms, we investigated the pattern and time course of cellular degeneration in the cochlea of conditional Cx26 (cCx26) null and Cx30 null mice. In cCx26 null mice, initial degeneration was observed around postnatal day 14 in outer hair cells (OHCs) and supporting cells surrounding the OHCs. All cells in the middle turn organ of Corti were lost 1 month after birth, and degeneration gradually spread to the basal and apical turns. Most spiral ganglion (SG) neurons in the middle and basal turns disappeared in the first 3 months, whereas significant amounts of apical SG neurons survived. In the cochlea of Cx30 null mice, survival of most inner HCs, supporting cells, and SG neurons was observed for up to 18 months. The most severe degeneration was found in apical SG neurons and OHCs. OHC loss followed a slow time course and a base to apex gradient. Gross structures of the endolymphatic space and stria vascularis observed at the light microscope level were unchanged in either Cx null mouse models. This study revealed that cellular degeneration in the cochlea of cCx26 null mice was dramatically more rapid and widespread than that observed in Cx30 null mice. The radically different pathogenesis processes displayed by cCx26 and Cx30 null mice suggest heterogeneous underlying deafness mechanisms, despite co-assembly of Cx26 and Cx30 in forming gap junctions in the cochlea.

Atoh1-lineal neurons are required for hearing and for the survival of neurons in the spiral ganglion and brainstem accessory auditory nuclei

Atoh1 is a basic helix-loop-helix transcription factor necessary for the specification of inner ear hair cells and central auditory system neurons derived from the rhombic lip. We used the Cre-loxP system and two Cre-driver lines (Egr2(Cre) and Hoxb1(Cre)) to delete Atoh1 from different regions of the cochlear nucleus (CN) and accessory auditory nuclei (AAN). Adult Atoh1-conditional knock-out mice (Atoh1(CKO)) are behaviorally deaf, have diminished auditory brainstem evoked responses, and have disrupted CN and AAN morphology and connectivity. In addition, Egr2; Atoh1(CKO) mice lose spiral ganglion neurons in the cochlea and AAN neurons during the first 3 d of life, revealing a novel critical period in the development of these neurons. These new mouse models of predominantly central deafness illuminate the importance of the CN for support of a subset of peripheral and central auditory neurons.

Novel drug delivery systems for inner ear protection and regeneration after hearing loss

BACKGROUND

A cochlear implant, the only current treatment for restoring auditory perception after severe or profound sensorineural hearing loss (SNHL), works by electrically stimulating spiral ganglion neurons (SGNs). However, gradual degeneration of SGNs associated with SNHL can compromise the efficacy of the device.

OBJECTIVE

To review novel drug delivery systems for preserving and/or regenerating sensory cells in the cochlea after SNHL.

METHODS

The effectiveness of traditional cochlear drug delivery systems is compared to newer techniques such as cell, polymer and gene transfer technologies. Special requirements for local drug delivery to the cochlea are discussed, such as protecting residual hearing and site-specific drug delivery for cell preservation and regeneration.

RESULTS/CONCLUSIONS

Drug delivery systems with the potential for immediate clinical translation, as well as those that will contribute to the future of hearing preservation or cochlear cellular regeneration, are identified.

Interactive roles of fibroblast growth factor 2 and neurotrophin 3 in the sequence of migration, process outgrowth, and axonal differentiation of mouse cochlear ganglion cells

A growth factor may have different actions depending on developmental stage. We investigated this phenomenon in the interactions of fibroblast growth factor 2 (FGF2) and neurotrophins on cochlear ganglion (CG) development. The portions of the otocyst fated to form the CG and cochlear epithelium were cocultured at embryonic day 11 (E11). Cultures were divided into groups fed with defined medium, with or without FGF2 and neurotrophin supplements, alone or in combination, for 7 days. We measured the number of migrating neuroblasts and distances migrated, neurite outgrowth, and axonlike processes. We used immunohistochemistry to locate neurotrophin 3 (NT3) and its high-affinity receptor (TrkC) in the auditory system, along with FGF2 and its R1 receptor, at comparable developmental stages in vitro and in situ from E11 until birth (P1) in the precursors of hair cells, support cells, and CG cells. Potential sites for interaction were localized to the nucleus, perikaryal cytoplasm, and cell surfaces, including processes and growth cones. Time-lapse imaging and quantitative measures support the hypothesis that FGF2 alone or combined with neurotrophins promotes migration and neurite outgrowth. Synergism or antagonism between NT3 and other factors suggest interactions at the receptor level. Formation of axons, endings, and synaptic vesicle protein 2 were increased by interactions of NT3 and FGF2. Similar experiments with a mutant overexpressor for FGF2 suggest that endogenous FGF2 supports migration and neurite outgrowth of CG neuroblasts as well as proliferation, leading to accelerated development. The findings suggest interactive and sequential roles for FGF2 and NT3.

Neurotrophins and electrical stimulation for protection and repair of spiral ganglion neurons following sensorineural hearing loss

Exogenous neurotrophins (NTs) have been shown to rescue spiral ganglion neurons (SGNs) from degeneration following a sensorineural hearing loss (SNHL). Furthermore, chronic electrical stimulation (ES) has been shown to retard SGN degeneration in some studies but not others. Since there is evidence of even greater SGN rescue when NT administration is combined with ES, we examined whether chronic ES can maintain SGN survival long after cessation of NT delivery. Young adult guinea pigs were profoundly deafened using ototoxic drugs; five days later they were unilaterally implanted with an electrode array and drug delivery system. Brain derived neurotrophic factor (BDNF) was continuously delivered to the scala tympani over a four week period while the animal simultaneously received ES via bipolar electrodes in the basal turn (i.e., turn 1) scala tympani. One cohort (n=5) received ES for six weeks (i.e., including a two week period after the cessation of BDNF delivery; ES(6)); a second cohort (n=5) received ES for 10 weeks (i.e., a six week period following cessation of BDNF delivery; ES(10)). The cochleae were harvested for histology and SGN density determined for each cochlear turn for comparison with normal hearing controls (n=4). The withdrawal of BDNF resulted in a rapid loss of SGNs in turns 2-4 of the deafened/BDNF-treated cochleae; this was significant as early as two weeks following removal of the NT when compared with normal controls (p<0.05). Importantly, there was not a significant reduction in SGNs in turn 1 (i.e., adjacent to the electrode array) two and six weeks after NT removal, as compared with normal controls. This result suggests that chronic ES can prevent the rapid loss of SGNs that occurs after the withdrawal of exogenous NTs. Implications for the clinical delivery of NTs are discussed.

An animal experimental model of auditory neuropathy induced in rats by auditory nerve compression

Several animal models of auditory neuropathy (AN) have been produced by employing pharmacological agents to damage auditory neurons or hair cells selectively. The specificity of pharmacological lesions is generally assessed by observation of visible structural damage but it is difficult to localize the delivery, which could lead to functional side effects in other anatomical structures. Although genetic analyses of human AN patients have provided important information on the pathophysiology of AN, specific genetic defects have not been fully correlated with functional deficits in the auditory nervous system. To address this problem, we compressed rat auditory nerves to assess neural degeneration for up to 35 weeks. The method produced a good model of auditory neuropathy, including profound deterioration of the auditory brainstem response and preservation of both cochlear microphonics and distortion product otoacoustic emissions. Histological examination revealed that in spite of profound degeneration of the auditory nerve, the hair cells remained intact. The model provides a complementary alternative to those based on pharmacological lesions and genetic analyses of AN patients and should allow analysis of the pathophysiology of auditory neuropathy with less risk of the results being confounded by unknown deficits in other cell types.

Glial cell line-derived neurotrophic factor and antioxidants preserve the electrical responsiveness of the spiral ganglion neurons after experimentally induced deafness

Cochlear implant surgery is currently the therapy of choice for profoundly deaf patients. However, the functionality of cochlear implants depends on the integrity of the auditory spiral ganglion neurons. This study assesses the combined efficacy of two classes of agents found effective in preventing degeneration of the auditory nerve following deafness, neurotrophic factors, and antioxidants. Guinea pigs were deafened and treated for 4 weeks with either local administration of GDNF or a combination of GDNF and systemic injections of the antioxidants ascorbic acid and Trolox. The density of surviving spiral ganglion cells was significantly enhanced and the thresholds for eliciting an electrically evoked brain stem response were significantly reduced in GDNF treated animals compared to deafened-untreated. The addition of antioxidants significantly enhanced the evoked responsiveness over that observed with GDNF alone. The results suggest multiple sites of intervention in the rescue of these cells from deafferentation-induced cell death.

Neurotrophic factors and neural prostheses: potential clinical applications based upon findings in the auditory system

Spiral ganglion neurons (SGNs) are the target cells of the cochlear implant, a neural prosthesis designed to provide important auditory cues to severely or profoundly deaf patients. The ongoing degeneration of SGNs that occurs following a sensorineural hearing loss is, therefore, considered a limiting factor in cochlear implant efficacy. We review neurobiological techniques aimed at preventing SGN degeneration using exogenous delivery of neurotrophic factors. Application of these proteins prevents SGN degeneration and can enhance neurite outgrowth. Furthermore, chronic electrical stimulation of SGNs increases neurotrophic factor-induced survival and is correlated with functional benefits. The application of neurotrophic factors has the potential to enhance the benefits that patients can derive from cochlear implants; moreover, these techniques may be relevant for use with neural prostheses in other neurological conditions.

Neurotrophin and Trk neurotrophin receptors in the inner ear of Salmo salar and Salmo trutta

Neurotrophins (NTs) and their signal transducing Trk receptors play a critical role in the development and maintenance of specific neuronal populations in the nervous system of higher vertebrates. They are responsible for the innervation of the inner ear cochlear and vestibular sensory epithelia. Neurotrophins and Trks are also present in teleosts but their distribution in the inner ear is unknown. Thus, in the present study, we used Western-blot analysis and immunohistochemistry to investigate the expression and cell localization of both NTs and Trk receptors in the inner ear of alevins of Salmo salar and Salmo trutta. Western-blot analysis revealed the occurrence of brain-derived neurotrophic factor (BDNF) and neurotrophin-3 (NT-3), but not nerve growth factor (NGF), as well as all three Trk receptors, i.e. TrkA, TrkB and TrkC, the estimated molecular weights of which were similar to those expected for mammals. Specific immunoreactivity for neurotrophins was detected mainly in the sensory epithelia. In particular, BDNF immunoreactivity was found in the maculae of the utricle and saccule, whereas NT-3 immunoreactivity was present in the sensory epithelium of the cristae ampullaris. As a rule the sensory epithelia of the inner ear lacked immunoreactivity for Trks, thus excluding possible mechanisms of autocrinia and/or paracrinia. By contrast, overlapping subpopulations of neurons in the statoacoustic ganglion expressed TrkA (about 15%), TrkB (about 65%) and TrkC (about 45%). The present results demonstrate that, as in mammals and birds, the inner ear of teleosts expresses the components of the neurotrophin-Trk system, but their roles remain to be elucidated.

Rebuilding lost hearing using cell transplantation

OBJECTIVE

The peripheral auditory nervous system (cochlea and auditory nerve) has a complex anatomy, and it has traditionally been thought that once the sensorineural structures are damaged, restoration of hearing is impossible. In the past decade, however, the potential to restore lost hearing has been intensively investigated using molecular and cell biological techniques, and we can now part with such a pessimistic view. In this review, we examine an important field in hearing restoration research: cell transplantation.

METHODS

Most efforts in this field have been directed to the replacement of hair cells by transplantation to the cochlea. Here, we focus on transplantation to the auditory nerve, from the side of the cerebellopontine angle rather than the cochlea.

RESULTS

Delivery of cells to the cochlea is potentially damaging, and nerve cells transplanted distally to the Schwann-glial transitional zone (cochlear side) may become inhibited when they reach the transitional zone. The auditory nerve is probably the most suitable route for cell transplantation.

CONCLUSION

The auditory nerve occupies an important position not only in neurosurgery but also in various diseases in other disciplines, and several lines of recent evidence indicate that it is a key target for hearing restoration. It is familiar to most neurosurgeons, and the recent advances in the molecular and cell biology of inner-ear development are of direct importance to neurorestorative medicine. In this article, we review the anatomy, development, and molecular biology of the auditory nerve and cochlea, with emphasis on the advances in cell transplantation.

Auditory hair cell explant co-cultures promote the differentiation of stem cells into bipolar neurons

Auditory neurons, the target neurons of the cochlear implant, degenerate following a sensorineural hearing loss. The goal of this research is to direct the differentiation of embryonic stem cells (SCs) into bipolar auditory neurons that can be used to replace degenerating neurons in the deafened mammalian cochlea. Successful replacement of auditory neurons is likely to result in improved clinical outcomes for cochlear implant recipients. We examined two post-natal auditory co-culture models with and without neurotrophic support, for their potential to direct the differentiation of mouse embryonic SCs into characteristic, bipolar, auditory neurons. The differentiation of SCs into neuron-like cells was facilitated by co-culture with auditory neurons or hair cell explants, isolated from post-natal day five rats. The most successful combination was the co-culture of hair cell explants with whole embryoid bodies, which resulted in significantly greater numbers of neurofilament-positive, neuron-like cells. While further characterization of these differentiated cells will be essential before transplantation studies commence, these data illustrate the effectiveness of post-natal hair cell explant co-culture, at providing valuable molecular cues for directed differentiation of SCs towards an auditory neuron lineage.

Site-specific interactions of neurotrophin-3 and fibroblast growth factor (FGF2) in the embryonic development of the mouse cochlear nucleus

Neurotrophins and FGF2 contribute to formation of the cochlea, but their roles in cochlear nucleus development are unknown. The effects of these factors may differ in the cochlea and cochlear nucleus, which may influence each other's development. It is important to analyze the effects of these factors on cellular structures at well-defined steps in the normal morphogenetic sequence. The present study used immunohistochemistry to localize factors in situ and to test hypotheses about their roles in an in vitro model. Specific antibody staining revealed that TrkC, the NT3 receptor, is present in neural precursors prior to embryonic day E11 until after birth. NT3 appeared in precursor cells during migration (E13-E15) and disappeared at birth. TrkC and NT3 occurred in the same structures, including growing axons, terminals, and their synaptic targets. Thus, NT3 tracks the migration routes and the morphogenetic sequences within a window defined by TrkC. In vitro, the cochlear nucleus anlage was explanted from E11 embryos. Cultures were divided into groups fed with defined medium, with or without FGF2, BDNF, and NT3 supplements, alone or in combinations, for 7 days. When neuroblasts migrated and differentiated, immunostaining was used for locating NT3 and TrkC in the morphogenetic sequence, bromodeoxyuridine for proliferation, and synaptic vesicle protein for synaptogenesis. By time-lapse imaging and quantitative measures, the results support the hypothesis that FGF2 promotes proliferation and migration. NT3 interacts with FGF2 and BDNF to promote neurite outgrowth, fasciculation, and synapse formation. Factors and receptors localize to the structural sites undergoing critical changes.

Expression of Prox1 during mouse cochlear development

We carried out an analysis of the expression of Prox1, a homeo-domain transcription factor, during mouse inner ear development with particular emphasis on the auditory system. Prox1 is expressed in the otocyst beginning at embryonic day (E)11, in the developing vestibular sensory patches. Expression is down regulated in maturing (myosin VIIA immunoreactive) vestibular hair cells and subsequently in the underlying support cell layer by E16.5. In the auditory sensory epithelium, Prox1 is initially expressed at embryonic day 14.5 in a narrow stripe of cells at the base of the cochlea. This stripe encompasses the full thickness of the sensory epithelium, including developing hair cells and support cells. Over the next several days the stripe of expression extends to the apex, and as the sensory epithelium differentiates Prox1 becomes restricted to a subset of support cells. Double labeling for Prox1 and cell-type-specific markers revealed that the outer hair cells transiently express Prox1. After E18, Prox1 protein is no longer detectable in hair cells, but it continues to be expressed in support cells for the rest of embryogenesis and into the second postnatal week. During this time, Prox1 is not expressed in all support cell types in the organ of Corti, but is restricted to developing Deiters' and pillar cells. The expression is maintained in these cells into the second week of postnatal life, at which time Prox1 is dynamically down regulated. These studies form a baseline from which we can analyze the role of Prox1 in vertebrate sensory development.

Exogenous BDNF rescues rat spiral ganglion neurons in vivo

OBJECTIVE

To determine if exogenous neurotrophins can prevent spiral ganglion neuron degeneration in the rat cochlea.

BACKGROUND

The loss of hair cells resulting in sensorineural hearing loss also leads to the secondary degeneration of spiral ganglion neurons. The effectiveness of cochlear implantation in patients with profound sensorineural hearing loss relies in part on the survival of spiral ganglion neurons; therefore, any therapy that can prevent or halt the loss of these neurons would be of potential clinical benefit. Previous research has shown that intracochlear infusion with neurotrophins can provide trophic support to SGNs in deafened guinea pigs. Whether this effect is seen in other species remains to be determined.

METHODS

After documenting the rate of spiral ganglion neuron degeneration after ototoxic deafening, we investigated the trophic effects of exogenous brain-derived neurotrophic factor (BDNF) on rat spiral ganglion neurons. The left cochleae of profoundly deafened rats were implanted with a drug delivery system connected to a mini-osmotic pump. BDNF or artificial perilymph was infused for 28 days; then the cochleae were prepared for histological study.

RESULTS

Treatment with BDNF led to a statistically significant increase in spiral ganglion neuron density and a highly significant increase in spiral ganglion neuron soma area compared with artificial perilymph-treated and untreated deafened cochleae.

CONCLUSION

The study has demonstrated the trophic advantage of exogenous BDNF in the mature rat cochlea and provides confidence that spiral ganglion neuron rescue after sensorineural hearing loss with exogenous BDNF may have clinical application.

Clinical application of neurotrophic factors: the potential for primary auditory neuron protection

Sensorineural hearing loss, as a result of damage to or destruction of the sensory epithelia within the cochlea, is a common cause of deafness. The subsequent degeneration of the neural elements within the inner ear may impinge upon the efficacy of the cochlear implant. Experimental studies have demonstrated that neurotrophic factors can prevent this degeneration in animal models of deafness, and can even provide functional benefits. Neurotrophic factor therapy may therefore provide similar protective effects in humans, resulting in improved speech perception outcomes among cochlear implant patients. There are, however, numerous issues pertaining to delivery techniques and treatment regimes that need to be addressed prior to any clinical application. This review considers these issues in view of the potential therapeutic application of neurotrophic factors within the auditory system.